Carbonates such as calcium carbonate have long been widely used in areas such as rubber, plastics and papermaking. Recently, many types of carbonates with high functionality have been developed and put into practical use for a variety of applications and purposes according to particle shapes and diameters.
Examples of crystal forms of the carbonate include calcite, aragonite and vaterite. Among these, aragonite is needle-like with superior hardness and coefficient of elasticity, and it may be used for versatile applications.
Commonly known manufacturing methods of the carbonates are, for example: (1) a reaction of a solution comprising carbonate ions with a chloride solution, and (2) a reaction of chloride with carbon dioxide. Also, Patent Literature 1 proposes a method regarding the method (1) to manufacture a needle-like aragonite-type carbonate, wherein a reaction of a solution comprising carbonate ions with a chloride solution takes place under ultrasonic irradiation. In addition, regarding a method of introducing carbon dioxide into water slurry of Ca(OH)2, Patent Literature 2 proposes a method to place a seed crystal of needle-like aragonite in Ca(OH)2 water slurry and grow the seed crystal only in a certain direction.
However, the carbonate obtained with the manufacturing method described in Patent Literature 1 is not only oversized of 30 μm to 60 μm in length but also has a wide distribution in the particle size; a carbonate controlled for a desired particle size may not be obtained. Furthermore, there is a problem that only a large particle with 20 μm to 30 μm in length may be obtained even with the manufacturing method of a carbonate described in Patent Literature 2.
On the other hand, the majority of optical-glass products such as glass lens and camera lens have been replaced by polymeric optical materials because of the lightness, superior mass productivity, and simplicity in application of molding technologies such as injection molding and extrusion molding. However, there is a problem that a product obtained from the molding of the common polymeric optical materials exhibits birefringence.
A polymeric optical material with birefringence does not cause a problem when it is applied to optical elements not requiring relatively high precision. However, optical materials with higher precision have been demanded recently, and birefringence is a big issue in case of, for example, a writable/erasable magneto-optical disk. A semiconductor laser beam is used for reading and writing this kind of magneto-optical disk; the existence of an optical element with birefringence in the disk itself and in the lens, for example, on the light path have an adverse effect on the precision in reading or writing.
Given this factor, Patent Literature 3 proposes a non-birefringent optical resin material comprising a polymeric material and an inorganic fine particle with different signs of birefringence from each other for the purpose of reducing the birefringence. The optical resin material of this proposal is obtained with a method called crystal doping, wherein, for example, a number of inorganic fine particles are dispersed in a polymeric resin, and the bonding chains in the polymeric resin and the inorganic fine particles are oriented in an approximately parallel direction by an external application of a molding force such as drawing to diminish the birefringence caused by the orientation of the bonding chain in the polymeric resin with the birefringence of the inorganic fine particles having an opposite sign.
As stated above, inorganic fine particles that may be used for crystal doping is essential in order to obtain a non-birefringent optical resin material through crystal doping. The inorganic fine particles are required to have a shape with a high aspect ratio such as a needle- or rod-like carbonate. Carbonate particles which do not influence the light transmittance, which is an important function as an optical material, is demanded. In addition, the average particle size of carbonate crystals need to be sufficiently small compared to the wavelength of a light source to reduce the effect of light scattering by particles as much as possible so that it does not affect the light transmittance.
However, there is a tendency that the effect of light scattering by particles increases with shorter wavelength of a light source. Also, there is an increasing percentage of products in which the optical information volume is increased by employing a light source with shorter wavelength of about 400 nm typified by a blue semiconductor laser beam. Therefore, reduced light scattering with smaller average particle diameter does not suffice. An engineering development that suppresses the reduction of transmittance in the visible short-wavelength region of around 400 nm is now strongly desired.
Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 59-203728
Patent Literature 2 Specification of the U.S. Pat. No. 5,164,172
Patent Literature 3 International Publication WO No. 01/25364